Sometimes, Earth’s climate system seems a lot like a Rube Goldberg machine—those zany marbles-and-mouse-traps sequences that circuitously complete some simple action. The ocean can interact with the atmosphere, which can interact with ecosystems on land, which can turn back and affect the atmosphere, and end up interacting with the ocean again. It can seem too complex to keep track of at first, but scientists have become quite familiar with many well-worn tracks in this climate contraption.
Records of climate during the last “ice age” show us a number of these crazy connections. When the planet was around 5°C colder and ice sheets covered large areas of the Northern Hemisphere, the climate featured some impressive fluctuations that aren’t possible in today’s warmer world. One of those was a periodic cycle of colder periods called “stadials” that each lasted for hundreds of years.
When the cold periods caused ice sheets in North America and Europe to expand sufficiently, they sometimes spawned sudden and massive outpourings of Atlantic icebergs called “Heinrich events.” These events are known from seafloor sediment cores, where you can find layers of pebbles and rocks that can only travel the open ocean by being trapped in icebergs.
Climate records around the world capture interesting changes during these cold periods—and sometimes during the Heinrich iceberg events, specifically. A new study of Brazilian cave stalagmites led by Fluminense Federal University’s Nicolás Stríkis finds that the Rube Goldberg output of the Heinrich events in eastern South America was stronger monsoon rains.
As cave stalagmites and stalactites grow, they can add new layers like the rings of a tree. As with tree rings, these layers can hold chemical records of climate change. The oxygen in this cave rock partly comes from rainwater that percolates through the soil and drips from the cave ceiling. The as isotopes of oxygen this water contains will change with climate conditions, making the layers a historical record.
Making over 6,000 measurements from several caves in Brazil, the researchers produced a record that goes back 85,000 years. In this area, the oxygen isotope signature of rainwater—which falls almost entirely during the summer monsoon season—changes as more and more rain falls. So later in a heavy monsoon season, or after several years of above-average rain, rainwater contains less oxygen-18 than it does in drier times.
Going back to the ice age climate over 15,000 years ago, the cave records showed significant drops in oxygen-18 at certain times. Those drops lined up with the timing of the Heinrich iceberg events in the North Atlantic. So whenever the northern ice sheets let loose a torrent of icebergs, the monsoon rains in eastern South America increased.
Why would that happen? The researchers’ proposed linkage gets pretty complex, but it actually fits in with our understanding of climate patterns in other regions. Releasing a flotilla of melting icebergs into the Atlantic makes the seawater a little less salty in the far north, and fresher seawater helps sea ice grow. The southward expansion of sea ice has a sort of “domino” effect, pushing cold water a little further south, which pushes the usual atmospheric high pressure south, as well.
The final domino is the calm boundary between Northern Hemisphere and Southern Hemisphere winds, which wanders north and south of the equator as the seasons change. The position of this boundary controls monsoon weather across the tropics. Shifting the boundary south a bit would bring heavier monsoon rains to the region where the cave records came from—all because of a bunch of icebergs 4,000 miles away.
This long-distance connection could actually help explain an interesting little tidbit seen in the ice cores: during the Heinrich iceberg events, atmospheric methane tends to briefly spike a bit. Because tropical wetlands are major sources of methane, boosting monsoon rains in the right regions would expand wetlands and produce some extra methane.
That’s how interconnected the Earth’s systems are. If you want to know what was going on in the skies above South America 16,000 years ago, just ask some Greenland ice, seafloor mud, and a Brazilian cave.